This invention relates generally to synchronization signal detection in a television receiver, and more particularly relates to a microcomputer-controlled system and method for detecting the presence of valid vertical sync pulses.
The signal received by a television receiver incorporates two primary components, a composite video signal and a composite synchronization signal. The video signal includes picture information such as luminance and chrominance signals. The composite synchronization pulse includes pulse-like signals occurring both at the horizontal and vertical scan rates which are interspersed between the scan interval picture component in the period generally referred to as the retrace interval. In addition, these sync pulse signals coincide in time with blanking signals used to blank out the video display during retrace.
The sync pulse signals are stripped from the blanking signals by means of a sync pulse separator circuit and applied to deflection circuitry in controlling the horizontal and vertical scanning of the television receiver's cathode ray tube. Because the sync pulses possess amplitudes in excess of the picture information signal level, sync separator circuits generally employ an amplitude, or threshold, responsive circuit to separate the sync information from the remainder of the signal. Most sync separators utilize a pulse-clamping technique in which a voltage proportional to sync pulse amplitude is stored as a reference bias. Also utilized is a clipper circuit responsive to the clamp voltage level which conducts only during a selected portion of the sync signal to the exclusion of video signal components. The output of this clipper circuit is then generally amplified and formed into appropriate, limited digital-type signals which are easily utilized in the synchronous driving of deflection circuitry.
Various factors in the video signal transmission environment increase the difficulty of synchronization pulse detection. These factors can produce excessive sync pulse amplitude variations, loss of low or high frequency components, imposition of white noise on the sync signal or the super-position of impulse noise on the incoming sync pulse. All of these phenomena either make sync pulse detection more difficult or increase the possibility of erroneous sync. For example, in a weak electrical field, or one containing a high noise level, vertical sync pulses may become broken up, or cracked, which may result in their non-detection by synchronization circuitry. Also, conventional sync separators require a signal level of at least 25% of the normal sync amplitude. However, with the composite video signal at a level of only 2-3 volts the sync separator circuit must be able to detect sync pulse amplitudes of 200 millivolts or less. This environment places high signal detection operating criteria on vertical synchronization detection circuitry.
One approach to the extraction of a vertical sync pulse from a composite sync signal is disclosed in U.S. Pat. No. 3,925,613. This approach is intended primarily to avoid non-synchronization problems caused by a noisy electrical field resulting in a cracked vertical sync pulse component which, in an extreme case, may be partially lost. The invention described therein involves the broadening of the widths of the input pulses to an extent at least larger than the width of the respective splits between the vertical sync pulses in converting the vertical sync pulse divided by noise-produced splits into a continuous form. Thus, this invention is capable of accurately detecting a sync signal in a noisy environment when the sync signal is present but is equally susceptible to false, or inaccurate, sync in an extremely noisy environment where noise input pulses to the television receiver are close in time.
Another approach to sync pulse extraction is disclosed in U.S. Pat. No. 4,084,187, wherein is described a circuit which establishes the back porch level of the composite video wave form at a first voltage level and establishes the sync signal level at a second voltage level and extracts the latter from the former at a signal level midway between these two voltage levels. The extraction process involves several feedback loops, an AGC amplifier and numerous other amplifiers, comparators and filters which make this approach feasible for only very specialized sync processes. Indeed, the primary intended use of this invention is with video tape recorders where lines during a portion of the vertical timing interval are absent from the video wave form and the video wave form signal level is at the blanking level.
Still other approaches to vertical sync signal recovery from the composite sync signal are disclosed in U.S. Pat. Nos. 3,873,768 and 3,530,238 and 4,097,896. The first of these patents emphasizes and detects the amplitude portion of the vertical sync signal. The second patent is primarily involved with measuring pulse timing in picking the vertical sync pulse off of the composite sync signal. The last approach referred to above, described in U.S. Pat. No. 4,097,896, also involves sync pulse amplitude detection and the generation of the sync signal output when sync pulse amplitude exceeds a predetermined reference voltage level. Still another pulse, or timing, detection approach is disclosed in U.S. Pat. No. 3,619,497, which involves the generation of a vertical sync signal at the field frequency by producing a pulse with a leading edge defined by the trailing edge of the first and a trailing edge defined by the leading edge of the second of the pulses of the vertical synchronizing pulse group.
Therefore, it can be seen from the discussion of the above-referenced patents that the prior art makes use of only one characteristic of the vertical sync pulse, either amplitude or timing, in the vertical synchronization process. The present invention, however, utilizes both characteristics of the vertical sync pulse for sync detection purposes. In addition, a third pulse input, i.e., duty cycle, is utilized for even more accurate detection of the presence of a valid vertical sync input pulse in a television receiver.